HomeMy WebLinkAbout20 - Design Report - Allison Ph IV - Wastewater
WASTEWATER
DESIGN REPORT
FOR:
ALLISON SUBDIVISION
PHASE IV
BOZEMAN, MT
Prepared By:
Madison Engineering
895 Technology Drive, Suite 203
Bozeman, MT 59718
(406) 586-0262
JUNE 2020
ALLISON SUBDIVISION PHASE IV
WASTEWATER SYSTEM DESIGN REPORT
A. General
This design report will give an overview of the proposed sewer main extension for the 48.21 acre
development of Allison Subdivision Phase IV (Section 24, Township 02S, Range 5E, Plat J-
608A) located at the intersection of Arnold Street and S. 11th Avenue in Bozeman. The
developed area in Phase IV will be 22.58 acres, with approximately 25.46 acres remaining as
future phase(s). This report focuses on Phase IV as well as including calculations for full build
out of the remaining areas.
The report includes a description of the proposed sewer main and design parameters. The
proposed improvements are anticipated to be constructed during the summer of 2020. Once the
sewer mains are constructed and approved, the mains will become a part of the City of Bozeman
system.
The following references were used in the wastewater facilities design and capacity analysis:
a. COB Design Standards and Specifications Policy, 2004, Addendum #6
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS), Addendum #3
c. City adopted MPWSS and Adopted Addenda
d. COB Wastewater Facility Plan, 2015
e. Circular DEQ- 2
f. City of Bozeman Zoning Map
g. City of Bozeman Infrastructure Viewer
The procedures and parameters outlined in the COB Design Standards and Specifications Guide
were used in the design of the sewer infrastructure for the proposed development.
B. Capacity Analysis
The subdivision falls in the Spring Creek Drainage Basin. The capacity was calculated for full
buildout of the remaining phases of Allison Subdivision, approximately 96 acres, as well as for
the current Phase 4, about 21 acres. The effluent generated from the proposed subdivision will
flow north into the existing City of Bozeman sanitary sewer manhole located in Opportunity
Way, then east in an existing 8” main, eventually reaching the existing 10” main in S 3rd Ave.
The wastewater demands from the proposed subdivision were estimated based on a per acre basis
for zoned areas. The densities per zoned areas were obtained from Table V-1 of the City of
Bozeman Design Standards and Specifications for R-1, R-2 and R-3 zoning. City of Bozeman's
Design Standards specify an average daily flow rate of 730 gal/acre/day for R-1 development,
980 gal/acre/day for R-2 development, and 1,220 gal/acre/day for R-3 development. The
proposed sewer mains have been sized to flow at no more than 75-percent of full capacity at
peak hour conditions of full build out, and a Manning coefficient of 0.013 was used for PVC
pipe.
The average daily flow based on the estimated wastewater flows is shown in Table 1 below.
Table 1
Average Daily Flow Rates
Basin Allison
Sub Phase Zoning Area
(acres)
Wastewater
Loading
(gpd/acre)
Average
Flow Rate
(gpd)
1 4
R-1 10.88 730 7,942
R-2 5.09 980 4,988
R-3 4.88 1,220 5,954
2 Future R-2 6.38 980 6,252
3 Future R-1 18.86 730 13,768
4 Future R-1 47.87 730 34,945
Total 93.96 73,849
The infiltration was determined based on 150 gpd/acre. A peaking factor for the entire
subdivision including future phases was calculated based on the following formula:
Qmax / Qave = (18 + P0.5) / (4 + P0.5) (P = Population/thousands)
Based on a total flow of 73,849 gpd and a flow rate of 65 gpcd, the total equivalent population is
1,136 persons. With this equivalent population, the peaking factor is 3.76. The calculated
peaking factor is included in the appendix of this report. The peak flow rate anticipated from the
development at full build out based on an average daily flow of 73,849 gallons per day, plus an
infiltration rate of 150 gpd/acre is:
Peak Flow Rate = (Average Flow Rate x Peaking Factor) + Infiltration
Peak Flow Rate = (73,849 gpd x 3.76) + (150 gpd/ac x 96.07 ac) = 292,083 gpd
= 203 gpm
= 0.452 cfs
The proposed Allison Subdivision Phase 4 plus future phase peak design flow is 0.452 cfs or 203
gpm.
The Allison Sub Phase 4 total design flow is 18,884 gpd, and at a flow rate of 65 gpcd, this
equates to 291 persons. With this equivalent population, the peaking factor is 4.08. The
calculated peaking factor is included in the appendix of this report. The peak flow rate
anticipated from the development at full build out based on an average daily flow of 18,884
gallons per day, plus an infiltration rate of 150 gpd/acre is:
Peak Flow Rate = (Average Flow Rate x Peaking Factor) + Infiltration
Peak Flow Rate = (18,884 gpd x 4.08) + (150 gpd/ac x 20.85ac) = 80,174 gpd
= 56 gpm
= 0.124 cfs
The Allison Subdivision Phase 4 peak design flow is 0.124 cfs or 56 gpm.
The sewer main size for the development will be an 8-inch, based on minimum required pipe
size and slope. An 8-inch PVC sewer main at 0.4% slope has a capacity of 0.70 cfs at ¾ pipe
depth which has adequate capacity for the proposed peak flow of 0.45 cfs effluent generated
from the 96-acre site. Calculations are provided in the appendix of this report.
C. Proposed Collection System
PVC pipe meeting COB specifications will be used for all the sewer mains. The maximum
manhole spacing will be 400 feet for the 8-inch main with a manhole barrel size of 48-inches.
Manholes will be constructed according to COB specifications. All sewer services will be 4-inch
PVC installed at 2% minimum grade. Sewer services will connect into the main with in-line
gasketed wyes. A 4-inch sewer service with a grade of 2.0% can convey approximately 60 gpm
at 50% capacity. The sewer main will be located within a 60-foot wide Right of Way and
maintain a minimum separation of 10-feet from the proposed public water main.
D. Downstream Capacity Analysis
It is anticipated that the City of Bozeman sanitary sewer system has enough capacity to
accommodate Allison Subdivision Phase IV. There is a potential downstream sewer constriction
in S. 3rd Avenue that would require flow monitoring for future phases of this subdivision. A
capacity letter from a City Engineer will be required prior to DEQ approval of the system.
E. Conclusion
As outlined above, the 8-inch sanitary sewer main serving Phase IV of Allison Subdivision will
have the capacity for the effluent generated the residential subdivision. In addition, there is
adequate capacity in the system to serve Allison Subdivision Phase IV. This project will require
the approval of the City Engineer and MDEQ.
Appendix
1. Wastewater Main Capacity Analysis
2. Peaking Factor Calculation
3. Sewer Drainage Basin Exhibit
Sewer Drainage Exhibit
EMEM
DRAINAGE BASIN 4 DRAINAGE BASIN 3
DRAINAGE BASIN 2
FUTURE PHASE FUTURE PHASE
FUTURE PHASE
DRAINAGE BASIN 1
PHASE IV
S.S. BASINSMADISON ENGINEERING
895 TECHNOLOGY BLVD, STE 203, BOZEMAN, MT 59718
PHONE (406) 586-0262
ALLISON SUBDIVISION PHASE 4
SEWER DRAINAGE BASINS
BOZEMAN,MT
1 inch =
0
SCALE
300 feet
600'150'300'
Pipe Capacity
Project Description
Friction Method Manning Formula
Solve For Discharge
Input Data
Roughness Coefficient 0.013
Channel Slope 0.40 %
Normal Depth 6 in
Diameter 8 in
Results
Discharge 0.70 ft³/s
Flow Area 0.28 ft²
Wetted Perimeter 1.40 ft
Hydraulic Radius 2 in
Top Width 0.58 ft
Critical Depth 0.39 ft
Percent Full 75.0 %
Critical Slope 0.00773 ft/ft
Velocity 2.48 ft/s
Velocity Head 0.10 ft
Specific Energy 0.60 ft
Froude Number 0.63
Maximum Discharge 0.82 ft³/s
Discharge Full 0.76 ft³/s
Slope Full 0.00333 ft/ft
Flow Type SubCritical
GVF Input Data
Downstream Depth 0 in
Length 0.00 ft
Number Of Steps 0
GVF Output Data
Upstream Depth 0 in
Profile Description
Profile Headloss 0.00 ft
Average End Depth Over Rise 0.00 %
Normal Depth Over Rise 75.00 %
Downstream Velocity Infinity ft/s
8-inch PVC Pipe Capacity
1/16/2014 8:28:10 PM
Bentley Systems, Inc. Haestad Methods Solution CenterBentley FlowMaster V8i (SELECTseries 1) [08.11.01.03]
27 Siemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 2of1Page
GVF Output Data
Upstream Velocity Infinity ft/s
Normal Depth 6 in
Critical Depth 0.39 ft
Channel Slope 0.40 %
Critical Slope 0.00773 ft/ft
8-inch PVC Pipe Capacity
1/16/2014 8:28:10 PM
Bentley Systems, Inc. Haestad Methods Solution CenterBentley FlowMaster V8i (SELECTseries 1) [08.11.01.03]
27 Siemons Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 2of2Page
Peaking Factor Calculation
Sewer Main Extension
Allison Subdivision Full Buildout
Peaking Factor
Total Flow = 73,849 gpd
Flow Rate = 65 gpcd
Equivalent Population = 1,136 people
=Q(max)= 18 + P
0.50 P = Population/thousands
Q(ave)4 + P
0.50
Peaking Factor =
Allison Subdivision Phase 4
Peaking Factor
Total Flow = 18,884 gpd
Flow Rate = 65 gpcd
Equivalent Population = 291 people
=Q(max)= 18 + P
0.50 P = Population/thousands
Q(ave)4 + P
0.50
Peaking Factor =
3.76
Peaking
Factor
Peaking
Factor
4.08